This invention relates generally to integrated circuits and more particularly to circuits used to reduce input and or output switching noise.
As is known in the art, an integrated circuit is comprised of a plurality of active and passive devices arranged to provide a circuit function on a common supporting semiconductive substrate. One type of integrated circuit is a so-called digital integrated circuit which is comprised of transistors used to form logic gates and logic functions for computer systems and other similar types of systems. A particular type of digital integrated circuit called an ASIC (application specific integrated circuit) generally has standardized circuits which can be customized to provide a particular digital function.
In general, with ASICs as well as other types of integrated circuits, there are signal and power connections made to the circuit. That is, DC power is delivered to the circuit to control transistors and other devices which process signals that are sent along signal lines. One problem associated with digital integrated circuits is that very dense integrated circuits, that is, those comprised of a large number of transistor devices and having associated a large number of signal pins, may produce transient voltages that affect power supply voltages delivered to logic circuits.
Generally ASICs are used to interface other logic devices to a system bus. The ASIC can include various system functional circuits such as, for example, a network bridge chip to interconnect two different buses such as a system bus and a network bus as well as other arrangements. One such circuits is a bus interface circuit. The bus interface circuits generally comprise two types of circuits, receivers and drivers. Drivers are circuits which, in response to an input logic signal, provide an output logic signal onto a signal bus. Drivers require a relatively large amount of current in order to drive all of the modules which may be connected to the system bus. Receivers, on the other hand, generally require less current. They are used to sense the signal from the system bus and provide as an output a received signal having a particular logic state. This signal is then used by the internal logic circuits on the ASIC. With drivers and receivers, the circuits are coupled to the power supply connections made to the ASIC. As the signals are fed from or to these circuits, the switching action of the signals changing state causes the drivers or receivers to draw current for a short period of time from the power supply.
The commencement of drawing current from the power supply through an inductive connection may cause a transient drop in the magnitude of the power supply voltage until the current reaches a steady state condition. Conversely, discontinuance of drawing of current through an inductive connection will cause a transient boost in the magnitude of the power supply voltage until the current reaches a steady state condition. A capacitance is generally included between the power supply connection and the reference connection in order to minimize this transient voltage variation at that location.
Nevertheless, these transients in the on-chip power supply provide a certain power source noise characteristic which is undesirable for operation of the bus drivers and receivers as well as the digital logic circuits contained within the ASIC. In order to solve this problem one approach is to provide a large number of power pins for a given number of signal pins. On the ASIC there is a corresponding number of power pins, that is, supply voltage and return for a given number of signal pins. While this is satisfactory in some respects to reduce noise, one problem associated with this approach is that it minimizes the number of signal pins that can exist for a given size ASIC package.
A second problem with power connections on ASICs is that mutual inductance of magnetically coupled devices and package parasitic inductances can produce transients causing data transitions commonly referred to as "input noise" or "output noise" when adjacent input and/or output or power supply pins of the package have switching voltages thereon. That is, the mutual inductance associated between a given pin on the package and its magnetically coupled neighbors can induce a current into the pin which can cause a transition error, that is, it can cause a spike which may be misunderstood as a logical transition. A third problem with power supply connection on ASICs is that voltage transients can reduce ASIC component reliability and lifetime through voltage over-stress induced failure.
In the prior art, such input noise problems and reliability problems were addressed by providing a large number of package pins dedicated to power as described above as well as through judicious selection of pin sequence, layout and output selection.